In the field of high output power lasers, it is common to employ an unstable resonator. These normally require that the lasing medium be of a suitably high gain, for the simple reason that the significant output coupling of a simple two-dimensional circularly-symmetric unstable resonator is too high for efficient power generation from a modest gain laser, such as a CO.sub.2 laser.
One way of overcoming this is to use a hybrid slab waveguide/unstable resonator. Examples of these can be seen in Tulip U.S. Pat. No. 4,719, 639, Nishimae U.S. Pat. No. 5,048,048 and Opower U.S. Pat. No. 4,939,738, each of which is herein incorporated by reference. Tulip employs a one-dimensional unstable resonator parallel to the long cross-sectional dimension of a slab laser, and a waveguiding arrangement which serves to confine the beam in a perpendicular dimension. This serves to reduce the output coupling and improve the power extraction efficiency. Such systems can be applied to negative-branch unstable resonators such as Nishimae, and to positiveresonators such as Opower.
A limitation on the design of such resonators within high power applications is that in order to improve mode discrimination and beam quality, the unstable resonator magnification needs to be increased. Within the context of a modest-gain lasing medium such as CO.sub.2 lasers, this can only be achieved at the expense of over-coupling the laser output. This leads to a corresponding reduction in power extraction efficiency.
It is an object of the present invention to provide a laser which overcomes this limitation inherent to the above-described prior art.
It is also an object of the present invention to allow improvements in beam quality by increasing the unstable resonator magnification without causing a corresponding loss in power extraction efficiency.